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Improve EventCount used by the non-blocking threadpool.

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The current algorithm requires threads to commit/cancel waiting in order
they called Prewait. Spinning caused by that serialization can consume
lots of CPU time on some workloads. Restructure the algorithm to not
require that serialization and remove spin waits from Commit/CancelWait.
Note: this reduces max number of threads from 2^16 to 2^14 to leave
more space for ABA counter (which is now 22 bits).
Implementation details are explained in comments.
This commit is contained in:
Rasmus Munk Larsen
2019-02-22 13:56:26 -08:00
parent 0b25a5c431
commit 6560692c67
3 changed files with 110 additions and 93 deletions
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10
unsupported/test/cxx11_eventcount.cpp
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@@ -30,11 +30,11 @@ static void test_basic_eventcount()
EventCount ec(waiters);
EventCount::Waiter& w = waiters[0];
ec.Notify(false);
ec.Prewait(&w);
VERIFY(ec.Prewait());
ec.Notify(true);
ec.CommitWait(&w);
ec.Prewait(&w);
ec.CancelWait(&w);
VERIFY(ec.Prewait());
ec.CancelWait();
}
// Fake bounded counter-based queue.
@@ -112,7 +112,7 @@ static void test_stress_eventcount()
unsigned idx = rand_reentrant(&rnd) % kQueues;
if (queues[idx].Pop()) continue;
j--;
ec.Prewait(&w);
if (!ec.Prewait()) continue;
bool empty = true;
for (int q = 0; q < kQueues; q++) {
if (!queues[q].Empty()) {
@@ -121,7 +121,7 @@ static void test_stress_eventcount()
}
}
if (!empty) {
ec.CancelWait(&w);
ec.CancelWait();
continue;
}
ec.CommitWait(&w);